1. Field of the Invention
The present invention relates to a shutter device of a lens-fitted photo film unit which has a shutter blade openably covering an exposure opening and a shutter drive lever striking the shutter blade against a bias of a spring.
2. Background Arts
A lens-fitted photo film unit, hereinafter called film unit, is premised on making exposures of only a single photo filmstrip, so that the film unit is designed as simple as possible in order to reduce its manufacturing costs. For example, the film unit adopts a so-called strike-type shutter device. The strike-type shutter device comprises a shutter drive lever movable in cooperation with shutter release operation, and a shutter blade covering an exposure opening. The shutter drive lever strikes an end of the shutter blade and opens the exposure opening while moving from a charge position to a release completion position. While the exposure opening is opened, a photo film is exposed to light from a subject. This strike-type shutter device is so designed as to have a fixed shutter speed within a range from {fraction (1/150)}th to {fraction (1/100)}th second for the sake of stable operation.
The shutter device, however, is not suited for low-light photography. When photographing a low-light subject like a night view, the shutter device causes under-exposed photography because the shutter speed is too fast for the low-light photography. Slowing down the shutter speed is effective in eliminating the disadvantage, but it is difficult to actualize the slow shutter speed with a simple shutter structure. It is possible to slow down the shutter speed with adopting a technology used in general cameras. In such a case, however, another problem, that is, increase in the manufacturing costs occurs.
An object of the present invention is to provide a shutter device of a lens-fitted photo film unit which realizes a slow shutter speed with a simple structure and at low cost.
To achieve the above object, a shutter device according to the present invention comprises a shutter drive lever which strikes a shutter blade against a bias of a spring, and a speed governor which acts on the shutter drive lever to slow down the shutter speed. The shutter drive lever has a shutter speed adjustment portion, and the speed governor has a speed-governing member. An exposure opening is held fully open in a predetermined time because the shutter speed adjustment portion and the speed-governing member make contact with each other. The speed governor is movable between a first position in which the speed-governing member contacts with the shutter speed adjustment portion to slow down the shutter speed, and a second position in which the speed-governing member does not contact with the shutter speed adjustment member. The shutter drive lever is rotatable around a fixed pivot. So, the speed governor realizes the slow shutter speed by means of slowing down a rotational speed of the shutter drive lever.
In the first embodiment, the shutter speed adjustment portion is a shutter speed adjustment gear formed in a periphery of the shutter drive lever. The speed governor comprises an escapement wheel with integrally formed two gear wheels, a swingable anchor with two claws, and a mounting board on which the escapement wheel and the anchor are mounted. The claw of the anchor is engaged with one of the two gear wheels to slow down a rotational speed of the escapement wheel. The speed governor is movable between the first position and the second position in accordance with movement of the mounting board. When the speed governor is in the first position, the other gear wheel is engaged with the shutter speed adjustment gear, so that the rotational speed of the shutter drive lever becomes slow.
In the second embodiment, the shutter speed adjustment member is a ledge formed in a periphery of the shutter drive lever. The speed governor has a leaf spring. The rotational speed of the shutter drive lever becomes slow because an end of the leaf spring makes contact with the ledge.